Geochemical exploration



United States Patent 3,457,044 GEOCHEMICAL EXPLORATION John B. Davis and Henry F. Yarbrough, Dallas, Tex., assignors to Mobil Oil Corporation, a corporation of New York No Drawing. Filed Dec. 27, 1965, Ser. No. 516,672 Int. Cl. G01n 31/06, 31/08 US. Cl. 23-230 8 Claims ABSTRACT OF THE DISCLOSURE A geochemical exploration technique which involves the analysis of formation water for one or more saturated hydrocarbons having at least ten carbon atoms and preferably ten to thirty carbon atoms. The presence of such hydrocarbons in the formation water is indicative of the propinquity of subterranean petroleum deposits.

This invention relates to geochemical exploration for petroleum minerals and more particularly to a geochemical exploration method which involves the identification of possible petroleum reservoirs utilizing the presence of certain saturated hydrocarbons in subterranean waters.

Petroleum is found in commercial quantities in subsurface rock formations such as sandstones and limestones. The presence of a petroleum deposit in a subterranean formation is not ordinarily manifested by readily discernible indicia at the earths surface. Accordingly, various techniques have been evolved in exploring for petroleum. Among these are those which fall within the general classification of geochemical exploration.

In most geochemical exploration techniques, a search is made at or near the surface of the earth, or at sub terranean locations Within the earths crust, for components of petroleum, precursors of petroleum, or derivatives thereof. These procedures are based on the theory that these materials may have migrated from an underlying petroleum reservoir to or near the surface of the earth or to a subterranean position remote from the petroleum reservoir. Thus, where materials such as hydrocarbons Which are normally constituent components of petroleum are found, the presence of a subterranean reservoir of petroleum in the area is indicated.

One class of geochemical exploration techniques is based upon the analysis of waters which flow within the earths crust. These waters may be either formation waters which flow through sedimentary rock formations and which may have come into contact with a subterranean petroleum deposit, or surface waters, that is, waters which in general flow near the surface of the earth and in all likelihood have not contacted a subterranean petroleum deposit, although they may have contacted seeps from such deposits.

It has in the past been proposed to analyze such waters for various petroleum constituents. For example, one conventional technique involves the analyzing of either formation or surface water for the light paraflins such as methane, ethane, propane, and in some cases butane. This is based upon the theory that as the water traverses an oil or gas reservoir, or in the case of ground water passes through an area of underground seepage from an oil or gas reservoir, it will absorb a quantity of hydrocarbon vapors and carry these vapors to the surface either by way of natural flow as in the case of natural springs or when tapped by a well. Therefore, an analysis of the gases in such Waters should be of value in determining Whether these Waters have traversed underground hydrocarbon-bearing deposits or have otherwise come into contact with hydrocarbons from such deposits.

In another technique it has been proposed to analyze subterranean waters for their aromatic hydrocarbon content and particularly for their benzene content. This technique is based upon the theory that such aromatic hydrocarbons are relatively soluble in water as compared with certain other hydrocarbons and that as water traverses a subterranean petroleum deposit, it will selectively dissolve such aromatics.

While techniques such as those described above may be of value in determining the presence of hydrocarbon deposits in a general area, they are subject to serious limitations. For example, analysis of ground or formation Water for gaseous hydrocarbons or light liquid hydrocarbons such as propane and butane is subject to errors because of the relative volatile of these materials. In this regard, such hydrocarbons may be lost from the sample during handling and processing, thus leading to erroneous results. In order to guard against loss of these volatile materials from the sample, special measures must be taken in obtaining the sample and also in processing it for analysis. Another difficulty, particularly with regard to the analysis of subterranean waters for aromatics such as benzene, resides in the fact that petroleum as found in nature does not always contain such aromatics. Thus, the subterranean water found in a particular formation may have in fact traversed a petroleum deposit. However, an.

analysis of this Water for aromatic hydrocarbons would prove negative if the deposit traversed did not contain the particular aromatics analyzed for. Also materials such as aromatic hydrocarbons and the low molecular weight parafiins referred to above may be selectively adsorbed from the Water as it moves through subterranean strata. Thus, Water found in a porous formation may test negative with respect to aromatics since the aromatics once contained in the Water were adsorbed therefrom as it passed through interstitial passages Within the earths crust.

In accordance with the instant invention, there is provided a geochemical prospecting method which involves the analysis of formation waters for certain saturated hydrocarbons and which is alleviative of the above-noted limitations and disadvantages. More particularly, the instant invention involves analysis of formation waters for one or more saturated hydrocarbons having at least ten carbon atoms. That is, formation water is analyzed qualitatively and preferably quantitatively for one or more hydrocarbons such as alkanes, cycloalkanes, and naph thenes having ten or more carbon atoms. Examples of these compounds are normal decane, isodecane, butyl cyclohexane, pentyl cyclohexane, etc.

While saturated hydrocarbons having at least ten carbon atoms (sometimes referred to herein as heavy saturated hydrocarbons) are soluble to only a limited extent in water, and therefore are found in formation waters only in small concentrations, they offer an accurate index of the flow history of formation waters and indicate whether a formation water during the course of its flow through subterranean portions of the earths crust has in fact come into contact with subterranean petroleum deposits. In this regard, petroleum oil as found in nature, while it may not contain aromatics, always will contain one or more of the heavy saturated hydrocarbons. Moreover, these saturated hydrocarbons are adsorbed out on the faces of the interstitial passages of the earth to a much lesser extent than the aromatic hydrocarbons or the light saturated hydrocarbons. Therefore, if water in flowing through a subterranean formation has in fact come into contact with a subsurface petroleum deposit, it is highly likely that it will contain the above-described saturated hydrocarbons, whereas it may not contain certain aromatics such as benzene or lighter hydrocarbons such as methane, ethane, etc. Also, the instant invention offers an extremely important advantage in that special procedures in handling the samples to prevent loss of the index hydrocarbons to the atmosphere are not required. In this regard, it will be noted that the heavy saturated hydrocarbons are of relatively low volatility and are not nearly as likely to evaporate from the water samples as are the lighter paraffins such as butane.

For a better understanding of the present invention and the objects achieved thereby, reference may be had to the following detailed description.

In carrying out the present invention, one or more formation water samples are obtained from the earths crust and analyzed for at least one saturated hydrocarbon having at least ten carbon atoms. By the phrase formation water, as used herein and in the appended claims, is meant Water which in the course of its migration through the earths crust has existed in a subterranean location within a sedimentary rock formation associated with the relatively ancient sediments in which petroleum deposits are found. The quoted term is thus distinguished from socalled ground water which is found in the superficial portion of the earth and originated as meteorologic water such as rainfall. Samples may be obtained in some instances at the surface of the earth such as at the outcrop of a water-bearing formation, although in most cases samples will be obtained from one or more wells drilled into the formation.

The sample or samples may be analyzed only qualitatively in order to determine merely the presence of one or more heavy saturated hydrocarbons as an index of the propinquity of subterranean petroleum deposits. Preferably, however, a quantitative analysis will be carried out in order to determine a characteristic representative of the amount of such saturated hydrocarbons in the water sample. While, in accordance with the broadest aspect of the invention, analysis may be carried out with regard to only one of the heavy saturated hydrocarbons, it is preferred to analyze the water sample for a plurality of such hydrocarbons. More particularly, it is preferred to analyze the sample for heavy saturated hydrocarbons in the C to C range. While the sample may be analyzed for heavy saturated hydrocarbons having more than 30 carbon atoms, this usually will be of small benefit since formation waters normally contain little if any of these high molecular weight hydrocarbons. An analysis of the samples for their saturated hydrocarbon content may be carried out in any suitable procedure. However, care must be taken to employ a procedure which will yield accurate results since the heavy saturated hydrocarbon concentration of such samples normally will be relatively low.

In a preferred procedure for analyzing for heavy saturated hydrocarbons, a water sample is obtained and then treated to provide a concentrated extract containing the heavy saturated hydrocarbons. This is accomplished by running the water sample through an activated, adsorptive solid material on which hydrocarbons are selectively adsorbed relative to water. Activated carbon serves as an excellent selective adsorbent of hydrocarbons from water and adsorbs the heavy saturated hydrocarbons relatively quantitatively. Therefore, in the preferred analysis procedure, activated carbon is used as the adsorbent, although other activated materials may be used.

The sample obtained is flowed through a suitable column of activated carbon. In experimental Work regarding the present invention a l /z-foot long column having a diameter of about three inches and packed with about 250 grams of 4 to 10 mesh activated carbon and about 250 grams of 20 to 40 mesh activated carbon was found to be satisfactory. For a detailed description of a column similar to that used in this experimental work, reference may be had to Middleton et al., Carbon Chloroform Extract (CCE) in Water, Journal American Water Works Association, vol. 54, No. 2, February 1962. Any suitable amount of water sample may be flowed through the carbon column. However, in most cases it has been found desirable in practicing the invention to obtain a water sample in the amount of at least 1000 gallons and to flow the sample through the column at a rate of about /2 to gallon per minute. At least this quantity is preferred in order to insure that a truly representative sample is obtained and also to insure that the heavy saturated hydrocarbons with regard to which the sample is analyzed will be extracted from the water in measurable amounts.

The sample may be collected by any suitable technique. Where, as is the usual case, a sample is obtained from a well penetrating a subsurface water-bearing formation, the carbon column may be connected directly to the wellhead. Water produced from the well thus may be passed directly into the column.

After flowing the desired quantity of water through the column, the hydrocarbons adsorbed on the activated carbon are extracted by desorption with a solvent. In tests carried out in developing the instant invention, it has been found that recovery of heavy saturated hydrocarbons from the activated carbon or other surface-active material can be substantially increased by acidifying the carbon prior to desorption with a solvent. Therefore, it is preferred in accordance with one embodiment of the present invention to acidify the adsorptive material by contacting it with an acid prior to the solvent extraction step. Steps should be taken to insure good coverage of the carbon by the acid. This may be accomplished by removing the carbon from the column, drying it, and then spraying it with an acid. Any suitable acid may be used in the acidification step, although the acid chosen should be relatively insoluble in the solvent used for desorption. Preferably an inorganic acid is used since organic acids are relatively soluble in most solvents suitable for the extracting step. One percent hydrochloric acid used in an amount of about milliliters has been found to be satisfactory. Like the other quantities set forth herein, this amount of acid is based upon the 500- gram carbon column described above. If a significantly greater amount of carbon is used, it may be desirable to increase the quantities of acid used accordingly.

The extraction step may be carried out with a suitable solvent such as chloroform or carbon tetrachloride. Chloroform has proven to be satisfactory for this step and is preferred. Carbon may be placed in a suitable extraction vessel and then extracted continuously with two liters of chloroform for a period of about 48 hours. The chloroform extract thus obtained is analyzed for its heavy saturated hydrocarbon content. While any suitable procedure may be used for analysis of the chloroform extract, a technique involving silica-gel column chromatography, infrared spectrometry, and gas chromatography has proven satisfactory.

In this technique, the chloroform extract is concentrated to a volume of about 100 milliliters by distillation and then further dried until a hydrocarbon residue is obtained. The drying step may be carried out in a vacuum if desired. The hydrocarbon residue is dissolved in a suitable amount, e.g., 5 milliliters, of a solvent such as normal heptane and the resulting solution is added to a suitable silica-gel chromatographic column. For example, the chromatographic column may be a IO-millimeter diameter glass column packed with about grams of 100 to 200 mesh activated silica gel and prewet with 5 milliliters of normal heptane. The column then is eluted with milliliters of normal heptane in successive increments of 5 milliliters each and thereafter milliliters of carbon tetrachloride in successive increments of 5 milliliters each. The majority of heavy saturated hydrocarbons are re covered in the normal heptane eluate, although some saturated hydrocarbons also will be found in the carbon tetrachloride fraction. The carbon tetrachloride fraction also may contain a minor amount of aromatic hydrocarbons.

The chromatographic fractions thus obtained may be analyzed by infrared spectrometry for saturated hydrocarbons. This step is optional since it is carried out merely to verify the presence of saturated hydrocarbons in the water sample. In carrying out this step, the heptane and carbon tetrachloride solvents are removed from the fractions recovered from the chromatographic column by a suitable drying procedure such as by evaporation at C. under air. The remaining extract is dissolved in a suitable amount, for example, 0.2 milliliter, of a suitable solvent such as carbon disulfide. The resulting carbon disulfide solution then is analyzed in an infrared spectrophotometer equipped with optics suitable for scanning in the 2 to 15 micron region. As will be understood by those skilled in the art, infrared absorption at wave lengths of 3.4-3.5 and 13.8-13.9 microns, respectively, indicates the presence of saturated hydrocarbons in the sample.

Following the infrared spectrometry the carbon disulfide solvent is removed by evaporation at 40 C. under air and the remaining extract is dissolved in a suitable solvent such as benzene. The resulting benzene solution of saturated hydrocarbons then is qualitatively and quantitatively analyzed by gas chromatography. Any suitable chromatographic analysis apparatus may be employed for this purpose. In experimental work relating to the present invention, a gas chromatograph having a 24" x A" column packed with silicone gum rubber on diatomitic aggregates available under the trade name Chromosorb W has proven to be satisfactory.

In carrying out the chromatographic analysis, helium or nitrogen may be utilized as the carrier gas and the column temperature may be increased from an initial temperature of about C. to a maximum of about 280 C. at the rate of about 4.6 C. per minute. Thereafter, the temperature may be held at 280 C. until the heavy saturated hydrocarbons under investigation are eluted completely.

The chromatogram produced by the chromatographic analysis will indicate the heavy saturated hydrocarbons present in the water sample and also their relative concentrations. Each hydrocarbon appears on the chromatogram as a peak at a definite recording time, the area under each peak being proportional to the concentration of its respective hydrocarbon in the sample.

The above-described analytical procedure should not be considered to be determinative with regard to the absolute concentration of the heavy saturated hydrocarbons in the sample. The procedure probably does not recover all of the heavy saturated hydrocarbon content of the sample. However, it does give relatively accurate results with regard to the relative concentrations of the various hydrocarbons and thus yields an accurate representative characteristic of the heavy saturated hydrocarbon content of the sample.

In a preferred embodiment of the invention, a plurality of formation water samples are obtained from different locations in the earths crust. These samples then are analyzed for one or more heavy saturated hyrocarbons by the above-described or other suitable procedures in order to determine for each sample the relative concentration of such hydrocarbons. These relative concentrations then are correlated with each other and the locations in the earths crust of their respective sampling stations in order to ascertain the propinquity of subterranean petroleum deposits. This correlation may be accomplished by plotting the relative concentration for each sample at its respective location on a geographical map of the area surveyed. The heavy hydrocarbon concentrations for the samples are indicative of their proximity to a petroleum deposit traversed by the formation water with the station sample exhibiting the highest such concentration being indicated as the closest to the deposit.

After correlating the results of the sample analyses, one or more exploratory wells may be drilled into the region of an indicated petroleum deposit. In determining the position of this well, consideration should be given to the directional movement of the formation water under investigation as well as the relative concentrations of heavy saturated hydrocarbons for the respective samples. Therefore, in accordance with one embodiment of the invention the exploratory well is spaced from the station of the sample having the highest such concentration in a direction counter to the trend of water movement. For example, if the subterranean waters in the region under investigation are known to flow from north to south, the exploratory well should be drilled in a somewhat northerly direction relative to the sampling station exhibiting the highest relative concentration of heavy saturated hydrocarbons.

Having described specific embodiments of the instant invention, it will be understood that further modifications thereof may be suggested to those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claims.

What is claimed is:

1. In a method of geochemical exploration, the steps comprising:

obtaining at least one formation water sample from the earths crust, and

analyzing said water sample for at least one saturated hydrocarbon having at least ten carbon atoms, the presence of said hydrocarbon in said sample being indicative of the propinquity of subterranean petroleum deposits. 1

2. The method of claim 1 wherein said water sample is analyzed for a plurality of saturated hydrocarbons having in the range of ten to thirty carbon atoms.

3. In a method of geochemical exploration, the steps comprising:

obtaining at least one formation water sample from the earths crust,

contacting said Water sample with a solid activated adsorptive material on which hydrocarbons are selectively adsorbable relative to water, contacting said adsorptive material with an acid, extracting hydrocarbons adsorbed on said material by contacting said adsorptive material with a solvent for said hydrocarbons, and

analyzing said extracted hydrocarbons for a plurality of saturated hydrocarbons having in the range of ten to thirty carbon atoms, the presence of said hydrocarbons being indicative of the propinquity of subterranean petroleum deposits.

4. The method of claim 3 wherein said adsorptive material is activated carbon.

5. The method of claim 3 wherein said acid is an inorganic acid.

6. In a method of geochemical exploration, the steps comprising:

obtaining subterranean water samples from a plurality of locations in the earths crust,

quantitatively analyzing said samples for at least one saturated hydrocarbon having at least ten carbon 8 7 atoms to determine for each of said samples the i 8. The method of claim 6 wherein each of said water relative concentration of said at least one hydrosamples is analyzed for a plurality of saturated hydrocarcarbon in said each of said samples, and bons having in the range of ten to thirty carbon atoms.

correlating said relative concentrations With each other and the locations in the earths crust of their respec- 5 References Cited tive samples to ascertain the propinquity of sub- UNITED STATES PATENTS terranean petroleum deposits. 2 40 11 8/194 Kennedy 7. The method of claim 6 further comprising the step 2 712 93 7 955 Huckabay of drilling a well at a position spaced from the location of 2 767 320 10 195 coggeshall et the sample having the highest of said relative concentra- 10 tions in a direction counter to the trend of formation MORRIS WOLK Primary Exammer water movement within the earths crust. R. M. REESE, Assistant Examiner 

